When brackish water or sea water is demineralized, by methods which include reverse osmosis, distillation, ion exchange and electrodialysis, the feed stream generally requires pH adjustment. The waters almost always contain appreciable quantities of calcium, magnesium and bicarbonate ions, otherwise described as hardness and alkalinity. The pH of the feed streams as received is generally close to 8, which means that it contains little CO.sub.2 and is essentially saturated with calcium carbonate. This in turn means that any attempt to extract purified water will result in concentrating the remaining feed fluid, causing the calcium carbonate concentration to exceed saturation, which rapidly causes scaling of the equipment surfaces with adverse consequences.
Therefore it is customary to reduce the pH to approximately 6 or less by the addition of mineral acid, such as H.sub.2 SO.sub.4 or HCl. This produces free CO.sub.2 and insures that there is no carbonate ion in the feed. CO.sub.2 will then appear in the purified water in all devices. The purified water will, in any event, contain relatively low concentrations of calcium or bicarbonate. Thus it is characterized as an "aggressive" water or one which is quite corrosive to community and domestic piping systems and components. The pH of such a water will probably be in the range of 5-6. These waters are typically very low in hardness.
For storage and distribution the water must be conditioned or "stabilized" by reducing the CO.sub.2 and additionally raising the pH to approximately 8.
Conventionally this is accomplished by a forced draft decarbonator followed by lime injection on a controlled basis to raise the pH to 8. This involves an electric motor, float controls, lime slurry preparation, controlled feeding and pH regulation, for example. The cost, even for small systems, of such relatively sophisticated devices can be quite considerable. Such systems also impose a burden of operator attention and sophistication. There is a need, generally in remote and/or undeveloped areas, for compact systems, which are relatively simple and undemanding as to operator skill and attention, with the capability to utilize demineralization to convert sea water and brackish water to purified water at the rate of a few thousands of gallons per day.